The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
During combustion in a diesel engine, an air/fuel mixture is delivered through an intake valve to cylinders and is compressed and combusted therein. After combustion, pistons force the exhaust gas in the cylinders into an exhaust system. The exhaust gas may contain oxides of nitrogen (NOx) and carbon monoxide (CO).
Exhaust gas treatment systems may employ catalysts in one or more components configured for accomplishing an after-treatment process such as reducing nitrogen oxides (NOx) to produce more tolerable exhaust constituents of nitrogen (N2) and water (H2O). Reductant may be added to the exhaust gas upstream from an after-treatment component, such as a selective catalyst reduction (SCR) component, and, for example only, the reductant may include anhydrous ammonia (NH3), aqueous ammonia or urea, any or all of which may be injected as a fine mist into the exhaust gas. When the ammonia, mixed with exhaust gases, reaches the after-treatment component, the NOx emissions are broken down. A Diesel Particulate Filter (DPF) may then capture soot, and that soot may be periodically incinerated during regeneration cycles. Water vapor, nitrogen and reduced emissions exit the exhaust system.
To maintain efficient NOx reduction in the after-treatment component, a control may be employed so as to maintain a desired quantity of the reductant (i.e., reductant load) in the after-treatment component. As exhaust gas containing NOx passes through the after-treatment component, the reductant is consumed, and the load is depleted. A model may be employed by the control to track and/or predict how much reductant is loaded in the after-treatment component and to inject additional reductant as required so as to maintain an appropriate reductant load for achieving a desired effect such as reduction of NOx in the exhaust stream.
It has been noted that predictions for reductant loads on after-treatment components, such as SCR catalysts, can be inaccurate following a service regeneration event. Service regeneration of a DPF is often conducted with exhaust gas temperatures at elevated levels. Because of these increased temperatures, it is often necessary to maintain a flow of reductant through the injector(s), dosing valves, or other after-treatment hardware in order to prevent thermal damage.
While the reductant load model may track the quantity of reductant that is injected, the model may have difficulty determining how much of the reductant actually accumulates on the SCR catalyst. At the high temperatures associated with a service regeneration event, reductant may be carried out of the system with the exhaust stream or may be oxidized. Accordingly, it can be difficult to predict how much of the reductant injected during service regeneration is oxidized or otherwise consumed in the after-treatment component and how much may have survived and accumulated so as to contribute to the loading of the after-treatment component.
As a consequence, model estimates of ammonia load may be inaccurate, and may thus be rendered unreliable. In particular, experience has shown that following the occurrence of certain events, such as a DPF service regeneration event, load estimates based on models may deviate substantially from observed levels of NH3 load on the after-treatment component. Hence, diagnostic processes based on measurement and evaluation of NOx reduction efficiencies in the after-treatment component may produce erroneous results such as where more reductant is actually loaded on the after-treatment component than the diagnostic system assumes based on the inaccuracies in the model. Such conditions may cause NH3 slip, which may cause some cross-sensitive sensors to misinterpret the presence of NH3 as NOx. Similarly, where an actual NH3 load is substantially lower than the model estimate, the incorrect NH3 load can cause a poorer than expected NOx reduction efficiency to be assessed by the diagnostic system, potentially resulting in an incorrect diagnosis and invocation of remedial measures to be taken.
Accordingly, it is desirable to provide a system and method for selectively disabling NOx reduction efficiency diagnostics during time periods when the accuracy of reductant load predictions are assessed as being unreliable so as to avoid inappropriate initiation of remedial measures when such measures are not warranted.